1. Field of the Invention
The present invention relates to a television signal transmission system using TDMA (Time Division Multiple Access) technique, which transmit television signals from a plurality of transmitting stations to a plurality of receiving stations.
2. Description of the Related Art
TDMA technique is to transmit a TDMA signal on one carrier. The TDMA signal consists of a plurality of TDMA frames recurring at predetermined period. Each of these frames is time-divided into a plurality of channels. The channels are assigned to a plurality of transmitting stations in, for example, one-to-one correspondence. Each transmitting station inserts signals in the channel assigned to it, and transmit the signals to receiving stations. The TDMA technique is advantageous in two respects. First, it is versatile. Second, it helps to increase the use efficiency of the carrier.
Recently a system has been in consideration by present inventors which utilizes the TDMA technique, thereby to transmit television signals from a plurality of transmitting stations to a plurality of receiving stations. The system comprises transmitting stations, more receiving stations, and, for example, a communications satellite. A transponder is incorporated in the communications satellite which can relay a plurality of carriers having different frequencies. One of the carriers is assigned to all transmitting stations. Each TDMA frame of the TDMA signal transmitted from each transmitting station o this carrier is time-divided into a plurality of communication channels. More precisely, each transmitting station inserts television signals an every image frame in the channel which is assigned to the transmitting station, and transmit the television signal to the communication satellite. Each of the television signals consists of a video signal and a audio signal. The video signal is frequency-modulated before being transmitted from the transmitting station. The audio signal is converted to a digital signal and then transmitted from the transmitting station on a subcarrier which has been modulated by QPSK technique. The transponder incorporated in the communications satellite receives the television signals from the transmitting stations, insert them in the various channels, and transmits them to the receiving stations. Each receiving station receives the television signals from the satellite, which are inserted in the various channels, and selects the television signals relayed and inserted in the desired channel. The television signals, thus selected, are stored into an image memory. When necessary, these signals are read from the image memory and processed, thereby displaying a still picture or a strobe-effect sequence of pictures on a monitor screen.
In order to select the desired communication channel among many, each receiving station needs to obtain a receiving interval as to when television signals inserted in the desired channel are received by the receiving station. This interval can be given to the receiving station by two methods. The first method is to insert time slots into the channels of each TDMA frame, and channel discriminant codes are set in the time slots, so that these codes are transmitted to the receiving stations. The second method is to transmit channel discriminant signals from each transmitting station to the receiving stations through ground communication lines.
The first method has a problem. Since a time slot is inserted in each communication channel, the region of the channel for containing television signals is smaller than otherwise. Consequently, the television signals can carry but less data, and their quality is inevitably lower than in the case where no time slots are inserted in the communication channel. In order to maintain the quality of the television signals, the TDMA frame can be time-divided into less channels, whereby each channel has an excessive region for a time slot. If the TDMA frame is time-divided into less channels, however, the efficiency of transmitting television signals will decrease.
The second method also has a problem. When this method is employed, the signal-transmitting system requires a number of ground communication lines, and is inevitably a large-scale one. Every time new receiving stations are added to the system, new ground communication lines must be installed for the new receiving stations. Therefore, it is difficult, from an economical point of view, to set up new receiving stations in the system. Further, the transmitting stations must undergo complex control in order to minimize or eliminate the between the time at which each receiving station receives the television signals from the satellite and the time at which the receiving station receives the channel-identifying signals transmitted via the ground communication line.